1. Field of the Invention
The invention relates generally to a ring valve gate mold device. More specifically, the invention relates to a ring valve gate mold used to produce an annular (ring-shaped) resin product by injection molding.
2. Description of the Related Art
When an annular resin product (ring-shaped product such as a pulsar ring, a cage, or a gear) is produced by injection molding using a resin-molding mold, if a gate, through which molten resin is supplied into a cavity for forming the resin product, is a single point on the circumference of the annularly-formed cavity, a resin merging point called a weld is created by injection molding. This makes a part of the product extremely low in strength compared with the other parts of the product. Therefore, as described in Japanese Patent No. 4203903, a ring valve gate mold, in which a gate is opened over the entire circumference of the annularly-formed cavity, is used for molding an annular resin product by injection molding. In this way, creation of a weld is prevented. In the ring valve gate mold described in Japanese Patent No. 4203903, one mold, which is a stationary mold, and the other mold, which is a movable mold, are disposed such that their mating faces can be joined together or separated from each other. An annular cavity having a product shape is formed in the mating face of the movable mold. Molten resin is supplied through the gate that is opened over the entire circumference of the annular cavity. The molten resin is supplied to the gate through a supply passage formed into a cylindrical shape. Opening and closing the gate with a valve body disposed at the gate causes the molten resin supplied to the gate to be supplied into the cavity and to be blocked from flowing into the cavity. The valve body disposed at the gate is a tubular sleeve body, and is disposed so as to be movable in the axial direction to open and close the gate. The tubular sleeve body is disposed at a position radially inward of the wall faces of the cylindrical supply passage and the gate through which the molten resin is supplied into the cavity. Therefore, heaters that keep the molten resin in the supply passage always in a molten state are all disposed at positions radially outward of the supply passage and the gate.
The ring valve gate mold having such a configuration as described above has no problem in molding a ring-shaped resin product having a relatively small diameter. However, the mold has limitations in molding a product having a relatively large diameter. This is because, in the ring valve gate mold having the above-described configuration, it is difficult to dispose a movable large sleeve body at a position on the inner peripheral side of the supply passage because the tubular sleeve body, as a valve body for opening and closing the gate, is disposed at a position on the inner peripheral side of the molten resin supply passage.